The server power gradually tends to be miniaturized with high power density. In the field of servers, a power system is usually constructed by a rack and plural power supply units accommodated in the rack. If the power system has to increase the output power without changing the width of the original-size rack, more power supply units having smaller widths should be employed. However, the output part of the power supply unit needs enough power transmission area for mounting the output rectification devices including for example the isolation transformer, the rectification device, the filter circuit and the output current path. In addition, the power transmission area is increased in accordance with increase of the output current. Thus, in a power supply unit with low-voltage and high current output, the required power transmission area for larger current is regarded as a limitation to reduce the width of the power supply unit. Consequently, it is difficult to miniaturize the power supply unit and increase the power density.
In order to achieve the miniaturization and the high power density, the layout and arrangement of components disposed in the output rectifier circuit of the power supply unit are particularly important. In the conventional power supply unit, after the secondary winding of the transformer is completed, the secondary winding of the transformer is electrically connected to a metal-oxide-semiconductor field-effect transistor (MOSFET) of a synchronous rectifier (SR) and then electrically connected to an output filter circuit through the circuit board. By utilizing the above output rectifier circuit, the electrically-conductive path of rectification at the secondary side is longer, so that the line loss is increased. Moreover the output rectifier circuit at the secondary side includes a lot of components. If the layout of components is unreasonable, the rectification effect is affected adversely and the occupied space is increased at the same time. Consequently, it is difficult to miniaturize the power supply unit and increase the power density.